Provide a directory separator if needed when converting a relative to an absolute path

[sethfowler]

You can't run p4c without installing it on macOS right now. The cause is that when searching for the P4 include files, on macOS we attempt to use the approach where we treat argv[0] as a relative path, but we don't insert a directory separator. The result is that when you run ./p4c-bm2-ss, we act as if the compiler is located at a path like /Users/sfowler/p4c/build./p4c-bm2-ss, which is obviously wrong. This prevents us from finding the include files, and we can't compile anything. This patch solves this problem. The patch is agnostic as to whether getcwd() returns a path with a directory separator at the end; I'm not sure whether this is consistent across platforms.

[ChrisDodd]

This was just inadequately tested -- I suspect getcwd always returns a path without the trailing /, though the documentation doesn't say.

Probably we should be using _NSGetExecutablePath on OSX anyways.

[sethfowler]

Yeah, agreed on both counts. Might be nice to add a little header in lib/ at some point for platform-specific wrappers for this kind of case.